The present invention relates to a solder glass composition, preferably vitreous, adapted to seal various components together, such as sealing a glass surface to another surface including glass, metal, ceramic, and like surfaces. However, the present glass is especially suited for use as a resistive thin film in coating the interior of cathode ray tubes to reduce or suppress arcing.
While glass surfaces may be sealed by applying sufficient heat to adjoining surfaces to cause them to fuse together, the relatively high temperatures which are necessary to achieve fusion create conditions that are detrimental to the glass in that they may also cause deformation as well as permanent stresses which, at a later time, may result in breakage, either spontaneously or when the glass is subjected to physical or thermal shock.
Moreover, fusion sealing is not practical when delicate or sensitive materials are in relatively close proximity to the surfaces being joined, since the high temperatures may adversely affect such materials. One example is in the assembly of a cathode ray tube when a cathodoluminescent surface is deposited on the face plate of the tube and a cathode ray electronic gun is assembled within the funnel portion of the tube. The peripheral edge of the face plate is then placed in juxtaposition with the peripheral edge of the funnel and the edges are sealed together. If the seal is formed by subjecting the adjoining glass surfaces to a temperature sufficient to fuse the glass, such a temperature may adversely affect the cathodoluminescent surface.
To avoid the problems of fusion sealing, soldering galsses are used having a softening point considerably lower than the sealing temperatures of glass surfaces to be united. In this manner the surfaces are safely subjected to a much lower temperature that need be only sufficient to cause the solder glass to soften and flow into the space between the surfaces to form a durable seal between them upon cooling without detrimentally affecting adjoining parts. Preferably, the solder glass has a softening point which is comfortably within the temperature range in which other components of a product, such as a cathode ray tube, are assembled and fixed in position.
Another problem peculiar to cathode ray tubes is arcing within the tube. Arcing occurs in the electron gun area of a cathode ray tube and can damage both the electron gun and the electronic circuitry which is responsible for the operation of the gun. The problem becomes potentially more serious in view of the trend toward the use of higher operating potentials, up to 30 kv, to enhance the brightness of the picture. Contaminants within a cathode ray tube and especially particulate contaminants can cause arcing. For example, in one practice a highly conductive graphite film is deposited on the tube funnel. If the film does not have adequate scratch resistance and adhesion characteristics, particles of the film may break loose, contaminate the tube, and introduce arcing. Further, contamination can also occur from normal manufacturing procedures and from normal use.
It has been proposed to apply a resistive thin film on the inside of a cathode ray tube. U.S. Pat. No. 3,355,617 to Schwartz et al forms such a film comprised of iron and manganese oxide.
U.S. Pat. No. 4,092,444 to Killichowski discloses depositing a resistive thin film on internal surfaces of a cathode ray tube by pyrolysis of a liquid mixture of colloidal graphite and a heavy metal resinate to produce a film which is a mixture of graphite and the oxide of the metal. The metal resinate is a combination of tin and antimony resinate.
For purposes of normal solder glass, it is usually preferable for the glass to be controllably devitrified or crystallized. Devitrifiable solder glasses do not have the capability of re-softening at their original softening points after they have once devitrified and the parts tend to stay in place because of little vitreous flow. A devitrified solder glass also forms a stronger seal than a vitreous one. However, in the resistive films it has been found that vitreous films are preferable to crystalline ones because devitrification is not needed and hard to control when present. For instance very rapid devitrification produces poor flow and poor adherence.